1. Field of the Invention
The present invention relates to a semiconductor device in which device isolation is made by a trench formed in a semiconductor substrate, and a method of fabricating the same.
2. Description of the Related Art
Conventionally, device isolation using such a trench is carried out in the following manner.
As shown in FIG. 1A, a trench 11 is formed in a semiconductor substrate 12, and a thin oxide film 13 is formed on the inner wall of the trench 11. A nonoxide film 14 is then formed only on a device region 15.
Then, with the nonoxide film 14 used as a mask, the resultant structure is subjected to an oxidization treatment to form a thick oxide film 16 on the surface of the device region 15 except on a part thereof, as shown in FIG. 1B.
Thereafter, polycrystalline silicon is buried in the trench 11 where the oxide film 16 is formed, followed by flattening of the top surface of the polycrystalline silicon, deposition of a cap oxide film, and so forth, thereby providing the device isolation.
According to this conventional method, the wall of the trench 11 and the surface of the substrate 12 define almost right angles at an upper corner portion 17 of the trench 11, as shown in FIG. 1B. At the time the thick oxide film 16 is formed, therefore, thermal stress is concentrated on the corner portion 17 at the oxidization time, and so is stress originated from expansion of volume or the like. This causes dislocation in the oxide film 16 near the corner portion.
This dislocation impairs the device isolation characteristic and the characteristic of a device formed in the device region. For instance, if bipolar transistors are respectively formed in multiple device regions, the dislocation results in an increase in leak current between the collectors of the individual transistors and deterioration of the current amplification factor. In other words, since the presence of dislocation to a certain degree increases the recombination current, the device characteristic and the device isolation characteristic are deteriorated.